Deposition of pyrolytic material



July 22, 1969 B. L. ETTINGER 3,457,042

DEPOSITION OF PYROLYTIC MATERIAL Fild Dec. 2, 1966 INVENTOR.

Emce i. 'z'fmyer 1. yw United States Patent l 3,457,042 DEPOSITION OFPYROLYTIC MATERIAL Bruce L. Ettinger, Washington, Mich., assignor toGeneral Electric Company, a corporation of New York Filed Dec. 2, 1966,Ser. No. 598,793 Int. Cl. COlb 31/04 US. Cl. 23-2091 12 Claims ABSTRACTOF THE DISCLOSURE A method for the deposition of pyrolytic materialwherein the mandrel upon which deposition occurs is provided with atleast one line of weakness, preferably in the form of a groove on theexterior surface of the mandrel, such that cracking is initiated andpropagates at least initially along the line of weakness to assure anoptimum breakage pattern.

Background of the invention The: subject matter of the present inventionis a method and apparatus for froming articles by gas pyrolysisdeposition Within a tubular mandrel, the key feature of which is animproved method and means for assuring a good separation of the mandrelfrom the article after deposition without adverse effect on the article.Th invention has particular utility in the manufacture of pyrolyticgraphite and will be described in detail specifically with referencethereto; however, it will be understood that the invention, in itsbroader aspects, is applicable to the manufacture of other types ofpyrolytic articles, examples of which will be given hereinafter.

Pyrolytic graphite is anisotropic, and by reason of this fact and itsextremely high temperature resistance and also its nuclear properties,has a broad field of utility such, for example, as for lamp filaments,furnace linings, nuclear reactor moderators, rocket nozzles and re-entryheat shields. The latter two types of articles have become particularlyimportant in recent years and generally require that the pyrolyticgraphite article he of tubular or circular cross section. Pyrolyticgraphite is manufactured by pyrolysis, or thermal decomposition, ofcarbonaceous gas. Any of a wide variety of carbonaceous gases may beused, though in practice methane, either alone or in combination withhydrogen, is preferred. To make a tube or annular article of pyrolyticgraphite, for example, the carbonaceous gas is passed through atubular-shaped mandrel preferably formed from ordinary electrographitehaving a controlled relatively smooth inner surface finish, the mandrelbeing heated to a sufficiently high temperature to cause pyrolysis ofthe carbonaceous gas with resultant deposition of the pyrolytic graphiteon the interior wall of the tubular mandrel. The graphite deposits inlaminae'and the process is continued until the desired thickness isaccomplished. The mandrel having the deposited article thereon is thencooled, whereupon the mandrel cracks due to differences in coefficientsof thermal expansion between the mandrel and the deposited article. Theformed article is then separated from the mandrel.

It is generally recognized that one of the important factors in thesuccess of the pyrolytic process is the thickness of the mandrel. Itmust not be so thick that it imposes loads on the deposited articlecausing the latter to crack during cool-down. Yet, it must not be sothin as to allow an undesirable amount of creep during deposition or sothin as to subject the mandrel to premature breakage, both of whichfrequently lead to out-of-roundness of the deposited article. Goodpractice has established that when the mandrel is fabricated fromconventional electrographite, its thickness should be one to one and a3,457,042 Patented July 22, 1969 half times the thickness of the articleto be deposited thereon.

Unfortunately, however, regardless of how carefully the thickness of themandrel is chosen, it frequently happens, owing to differences in thecoefficients of thermal expansion of the deposited article and themandrel, that one or more axial cracks (i.e., cracks parallel to thelongitudinal axis of the mandrel) develop in the mandrel as the mandrel,with its associated deposited article, is cooled. Such axial crackscause the deposited article to be out-of-round with the maximum radiicorresponding to the locations adjacent to such axial failures. Inaddition, when a mandrel fails axially, such failure is often so violentthat, regardless of the use of various pretreatment techniques, seriousspalling of the deposited article occurs. is

Ideally, the stresses of cooling should ultimately cause the mandrel tofail in a generally crazed breakage pattern.

Such a breakage pattern does not distort the deposited article and,furthermore it facilitates the uniform separation of the depositedarticle and the mandrel. Under present practice, utilizing a mandrel ofuniform thickness equal to about one to one and a half times thethickness of the deposited article, the mandrel does at times break withthe desired crazed pattern. Yet all too often it breaks instead in theundesirable axial pattern. Thus, the pyrolytic graphite manufacturingprocess, using present state of the art mandrels, continues to result ina high scrap loss in the manufacture of such articles due to damageresulting from unwanted axial mandrel failures. The art is in need of asimple and economical method for insuring that the mandrel breaks, uponcooling, in a pattern which assures against damage to the depositedarticle and which enables a good separation of the mandrel from thearticle. The present invention fulfills this need.

Summary of the invention In accordance with the present invention, themandrel is provided with at least one linear frangible section, i.e., aline of weakness, preferably in the form of a groove on the exteriorsurface of the mandrel, which extends both axially and circumferentiallyon the mandrel such that when the stresses occur during cool-down,cracking is initiated and propagates at least initially along the lineof frangibility thereby assuring an optimum breakage pattern. In thepreferred embodiment of the invention, the mandrel is made thicker thanthe thickness that would normally be used and is provided with at leastone spiral groove from one end to the other in the exterior surfacethereof, the thickness of the grooved portion being that which wouldnormally be used for the mandrel. With such structure the mandrel hasincreased hoop strength to better assure against axial cracks during andsubsequent to the deposition due to its greater than normal thickness.Yet, the stresses developed during cooling, because they are at leastinitially released through the failure of the grooved portion, result ina breakage pattern which is ideal, the cracks occurring ultimately notonly along the spiral groove but also along lines between andintersecting the groove. Hence, by reason of the spiral groove, abreakage pattern resembling, and in fact superior to, the desired crazedpattern of conventional mandrels is attained and with assurance againstan undesirable axial breakage pattern.

Brief description of the drawing Other objects, features and advantagesof the invention will appear more clearly from the following detaileddescription of the preferred embodiment thereof made with reference tothe drawings in which:

FIGURE 1 is a sectional view of a furnace incorporating a mandrelembodying the invention, which mandrel has deposited thereon a pyrolyticgrapihte article;

FIGURE 2 is an isometric view of a portion of the mandrel of FIGURE 1before cooling occurs; and

FIGURE 3 shows the mandrel of FIGURES 1 and 2 after cooling hasoccurred.

Description of the preferred embodiment Referring now to FIGURE 1, theapparatus shown comprises a generally cylindrical casing havingenclosure plate 12 which is removably secured as by bolts or a suitablehinge and latch. A viewing window 13 enables inspection of thedeposition operation within the casing and also viewing with an opticalpyrometer. A body of the insulating material 14, such as carbon black,defines an inner cylindrical chamber, the walls of which are formed by agraphite cylinder 18 and top and bottom graphite plates 20 and 22respectively. An induction heating coil 24 surrounds the insulatedmaterial 14, the graphite cylinder 18 functioning as the susceptorwhereby intense heat is generated within the cylinder 18 by reason ofthe passage of current through the induction coil 24.

Extending through the heating chamber defined by the cylinder 18 and itsend plates is a mandrel assembly 25. An opening in plate 22 accommodatesan inlet tube 26 for the flow of carbonaceous gas into and through themandrel, the upper end of the mandrel assembly being open to theinterior of the casing 10 whereby the nondeposited products of thepyrolysis of the carbonaceous gas can exit through the outlet tube 28.Hence, in operation, the carbonaceous gas, such as methane or a mixtureof methane and hydrogen, is admitted through tube 26 to the interior ofthe mandrel assembly which is intensely heated by the heat generated bythe cylinder 18. Pyrolysis of the carbonaceous gas thereby occurs withresultant deposition of pyrolytic graphite on all of the interior wallsof the mandrel assembly thereby forming article 40, the hydrogen andother gaseous pyrolysis products being withdrawn from the chamberthrough the out let tube 28. As is well known in the art, temperatureson the order of 1200 C. to 2500 C. can be used to cause the pyrolysisreaction.

In accordance with conventional practice, the mandrel assembly comprisesa central tubular part 30, which constitutes the mandrel in which thedesired pyrolytic graphite article is deposited, and termination mandrelportions 36 and 38 separated from the part 30 by parts 32 and 34 whichprovide radially inwardly extending flanges. This enables the highquality deposit in the central portion of the assembly to be cleanlyseparated from the lower quality deposits in the top and bottomtermination portions of the mandrel assembly.

In accordance with the present invention, the mandrel 30 is provided, asby a cutting operation, with a spiral groove 42 on the outer surfacethereof. Preferably, the groove depth should be from 20% to 70% of thethickness of the mandrel. The ungrooved portion of the mandrel 30 is ofa thickness greater than 1.5 times, and more specifically, about twicethat of the article 40.

Instead of using only one groove, as shown, a plurality of spiralgrooves may be cut in the mandrel. Where a plurality of grooves is used,all may be of the Same lead, lefthanded or righthanded, or they may beof different leads such that they intersect. I have found that the mostdesirable form of spiral is one having a lead angle (i.e., the anglebetween the spiral, at any point thereon, and the longitudinal axis ofthe mandrel) of from about 45 to 80. Preferably, the groove or groovesshould circumscribe the mandrel at least once, from one end to the otherthereof, and it is most desirable that the distance along thelongitudinal axis of the mandrel, between one groove turn and the nextgroove turn (where a Single groove is used, such constituting the pitch)not be greater than a third of the length of the mandrel. When theseconditions cannot be satisfied by one spiral groove, two or more can beused so that such optimum conditions are satisfied.

Although a spiral groove shape is preferred, other groove shapes may beused if desired, the essential feature being that the groove extend in adirection which is askew to the longitudinal axis of mandrel 30, suchthat the line of the groove has an axial component and a circumferentialcomponent at substantially all points therealong.

Referring again to the apparatus show nin FIG. 1 and to its operation,after the pyrolytic graphite deposition is completed, mandrel 30 andarticle 40 are cooled. Since the mandrel has increased hoop strength byreason of the greater than usual thickness in the ungrooved portions,initial failure will occur in the spiral grooved portion. Cracks willthen ordinarily develop between the spirals, an example of the finalbreakage pattern being shown in FIG. 3. Hence, the mandrel develops,upon cooling, a breakage pattern resembling, and in fact superior to,the desired crazed pattern of conventional mandrels.

As previously pointed out, whereas the invention has its greatestutility and has been described specifically with reference to themanufacture of pyrolytic graphite articles, it will also find utility inthe manufacture of pyrolytic articles of other materials. Suchadditional materials include the refractory metals of Groups IV, V andVI of the Periodic Table and their carbides, borides, nitrides andsilicides such as those of hafnium, molybdenum, niobium, silicon,tantalum, tungsten, titanium and zirconium. As is well known in the art,the gas composition used for the pyrolysis reaction to form suchpyrolytic articles will vary depending upon the precise materialdesired. For example, the aforesaid metals may be pyrolyticallydeposited by using as starting materials the halogenated derivatives ofthe metals, whereas to form the nitrides carbides, for example, thestarting materials will consist of a mixture of the metal derivativesplus ammonia or carbonaceous gas, respectively. In its broader scope,therefore, the invention will find utility for the manufacture of anypyrolytic article wherein there exists the problem of separating themandrel from the deposited article while yet insuring that the mandrel,upon cooling, does not crack only along axial lines.

It will be understood that While the invention has been describedspecifically with reference to a particular embodiment thereof, variouschanges and modifications may be made, all within the full and intendedscope of the claims which follow. For example, instead of being a rightcylinder, as shown, the mandrel 30 can be of varying radius from one endto the other, the precise shape of the tubular mandrel being determinedby the shape desired for the article deposited thereon.

I claim:

1. A method for manufacturing an article of pyrolytic materialcomprising pyrolytically depositing said material on the interiorsurface of a tubular mandrel having at least one exterior groove whichextends in a direction having both axial and circumferential components,cooling the mandrel having the deposited pyrolytic material thereonwhereupon the mandrel cracks, and separating the pyrolytic material fromthe mandrel.

2. A method as set forth in claim 1 wherein said groove constitutes aspiral.

3. A method as set forth in claim 1 wherein said deposition is bypyrolysis of a carbonaceous gas forming a pyrolytic graphite article andwherein said mandrel is of electrographite.

4. A method as set forth in claim 2 wherein said spiral has a lead angleof from about 45 to 5. A method as set forth in claim 2 wherein saidspiral circumscribes said mandrel at least once from one end to theother thereof.

6. A method as set forth in claim 1 wherein the depth of said groove isfrom 20% to 70% of the thickness of the mandrel.

7. In apparatus for the deposition of a pyrolytic article comprising achamber, pyrolytic material feed means con nected to said chamber, atubular mandrel within said chamber upon an interior surface of whichthe pyrolytic material is deposited, and heating means surrounding saidmandrel,

the improvement in which the tubular mandrel has at least one exteriorgroove extending in a direction having both axial and circumferentialcomponents such that when stresses occur during cool-down, cracking isinitiated and propagates at least initially along said groove. 8.Apparatus as set forth in claim 7 wherein said groove is a spiral.

9. Apparatus as set forth in claim 7 wherein said mandrel is formed fromelectrographite.

10. Apparatus as set forth in claim 8 wherein said 15 spiral groove hasa lead angle of from about 45 to 80.

11. Apparatus as set forth in claim 8 wherein the spiral groovecircumscribes the mandrel at least once from one end to the otherthereof.

12. Apparatus as set forth in claim 8 wherein the depth of said spiralgroove is from 20% to 70% of the thickness of the mandrel.

References Cited UNITED STATES PATENTS 3,213,177 10/1965 Diefendorf23209.1 X 3,294,880 12/1966 Turkat 264-81 X 3,335,345 8/1967 Diefendorf117-46 X EDWARD J. MEROS, Primary Examiner US. Cl. X.R.

